1. Field of the Invention
This invention relates to a radiation image storage panel for use in a radiation image recording and reproducing techniques, in which phosphors are utilized. This invention also relates to a process for producing the radiation image storage panel.
2. Description of the Related Art
As techniques for obtaining radiation images for medical diagnosis and radiation images of various objects in a non-destructive mode and utilizing the radiation images for making medical diagnosis, flaw detecting inspections, and a like, radiography utilizing a combination of a silver halide photographic material (hereinbelow referred to simply as the photographic material) and an intensifying screen has heretofore been used in practice. With the radiography, radiation carrying image information of an object is irradiated to the intensifying screen, and a phosphor contained in the intensifying screen is excited by the radiation to emit near ultraviolet light or visible light. The emitted light impinges upon the photographic material, and a radiation image of the object is recorded on the photographic material The radiation image is utilized for making medical diagnosis, flaw detecting inspections, or the like. Specifically, the photographic material comprises a substrate and a silver halide emulsion layer, which is overlaid on one surface of the substrate, or silver halide emulsion layers, which are overlaid on opposite surfaces of the substrate. Also, an intensifying screen is brought into close contact with one surface of the photographic material, or intensifying screens are brought into close contact with opposite surfaces of the photographic material. The combination of the photographic material and at least one intensifying screen is exposed to radiation carrying image information of the object, and the radiation image is thereby formed.
In lieu of the conventional radiography described above, radiation image recording and reproducing techniques utilizing stimulable phosphors have heretofore been used in practice. The stimulable phosphors have the properties such that, when the stimulable phosphors having absorbed energy from radiation are exposed to an electromagnetic wave, such as visible light or infrared rays, the stimulable phosphors are stimulated to emit the energy as fluorescence. The radiation image recording and reproducing techniques are described in, for example, U.S. Pat. No. 4,239,968. The radiation image recording and reproducing techniques utilize a radiation image storage panel (referred to also as the stimulable phosphor sheet) provided with a stimulable phosphor. With the radiation image recording and reproducing techniques, the stimulable phosphor of the radiation image storage panel is caused to absorb radiation, which carries image information of an object or which has been radiated out from a sample, and thereafter the stimulable phosphor is exposed to an electromagnetic wave (stimulating rays), such as visible light or infrared rays, which causes the stimulable phosphor to produce the fluorescence (i.e., to emit light) in proportion to the amount of energy stored thereon during its exposure to the radiation. The produced fluorescence (the emitted light) is photoelectrically detected to obtain an electric signal. The electric signal is then processed, and the processed electric signal is utilized for reproducing a visible image of the object or the sample.
The radiation image recording and reproducing techniques have the advantages in that a radiation image containing a large amount of information can be obtained with a markedly lower dose of radiation than in the conventional radiography utilizing the radiation film and the intensifying screen. Therefore, the radiation image recording and reproducing techniques are efficient particularly for direct medical radiography, such as the X-ray image recording for medical diagnosis.
Basically, the radiation image storage panel utilized for the radiation image recording and reproducing techniques comprises a substrate and a stimulable phosphor layer overlaid on one surface of the substrate. In cases where the stimulable phosphor layer has self-supporting properties, the radiation image storage panel need not necessarily be provided with the substrate. Ordinarily, a transparent protective film is formed on the surface of the stimulable phosphor layer, which surface is opposite to the surface that stands facing the substrate. The transparent protective layer protects the stimulable phosphor layer from chemical deterioration or physical shocks.
The stimulable phosphor layer comprises a binder and stimulable phosphor particles dispersed in the binder. The stimulable phosphor has the properties such that, when the stimulable phosphor is caused to absorb radiation, such as X-rays, and is then exposed to the stimulating rays, the stimulable phosphor emits light in proportion to the amount of energy stored thereon during its exposure to the radiation. Therefore, when the radiation image storage panel is exposed to the radiation, which carries image information of an object or which has been radiated out from a sample, the stimulable phosphor layer of the radiation image storage panel absorbs the radiation in proportion to the dose of radiation, and a radiation image of the object or the sample is stored as an image of energy from the radiation on the radiation image storage panel. The radiation image storage panel is then exposed to the stimulating rays, and the image having been stored on the radiation image storage panel can be detected as the light emitted by the radiation image storage panel. The emitted light is detected photoelectrically to obtain an image signal, the image signal is processed, and the thus obtained processed image signal can then be utilized for reproducing the radiation image of the object or the sample as a visible image.
As described above, the radiation image recording and reproducing techniques are the advantageous image forming techniques. However, as in the cases of an intensifying screen employed in the conventional radiography, it is desired that the radiation image storage panel utilized for the radiation image recording and reproducing techniques has a high sensitivity and can yield an image of good image quality (with respect to sharpness, graininess, and the like).
Basically, the sensitivity of the radiation image storage panel depends upon a total light emission intensity of the stimulable phosphor, which is contained in the radiation image storage panel. The total light emission intensity depends upon light emission luminance of the stimulable phosphor itself and the content of the stimulable phosphor in the stimulable phosphor layer. In cases where the content of the stimulable phosphor in the stimulable phosphor layer is high, a high absorptivity is capable of being obtained with respect to the radiation, such as the X-rays. Therefore, in cases where the content of the stimulable phosphor in the stimulable phosphor layer is high, a high sensitivity is capable of being obtained, and the image quality (in particular, with respect to the graininess) is capable of being enhanced. In cases where the content of the stimulable phosphor in the stimulable phosphor layer is kept at a predetermined value, if the stimulable phosphor particles are packed closely in the stimulable phosphor layer, the thickness of the stimulable phosphor layer can be set to be thin. Therefore, in such cases, spreading of the stimulating rays due to scattering in the stimulable phosphor layer is capable of being minimized, and an image having a high sharpness is capable of being obtained.
As radiation image storage panels provided with a stimulable phosphor layer, in which a stimulable phosphor is packed closely, and processes for producing the radiation image storage panels, the applicant proposed radiation image storage panels, in which a void content in the stimulable phosphor layer has been reduced by performing compression processing on the stimulable phosphor layer, and processes for producing the radiation image storage panels. The proposed radiation image storage panels and the proposed processes for producing the radiation image storage panels are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 59(1984)-126299 and 59(1984)-126300.
With the proposed radiation image storage panels, the stimulable phosphor layer is subjected to the compression processing, and the density of the stimulable phosphor in the stimulable phosphor layer is set at a higher value than in conventional radiation image storage panels. As a result, an image having a high sharpness is capable of being obtained with the proposed radiation image storage panels. However, the proposed radiation image storage panels have the problems in that the stimulable phosphor is partially broken due to the compression processing, and the sensitivity becomes low. Therefore, research has heretofore been conducted to employ techniques for enhancing dispersibility of stimulable phosphor particles and minimizing the void content in the stimulable phosphor layer before the compression processing is performed.
As one of the techniques for improving the state of dispersion of the stimulable phosphor, a technique, in which a dispersing process is performed for a long time when a coating composition containing the stimulable phosphor is to be prepared. However, the technique for performing the dispersing process for a long time has the problems in that a considerable shearing force is exerted on the stimulable phosphor, and the characteristics of the stimulable phosphor are often lost. Also, the technique for performing the dispersing process for a long time has the problems in that a long time is required to produce the radiation image storage panel, and the working efficiency cannot be kept high.
Accordingly, research has heretofore been conducted to employ techniques for efficiently dispersing a stimulable phosphor such that a marked alteration may not be made on ordinary processes for producing a radiation image storage panel. As one of such techniques, a technique for performing surface treatment on a stimulable phosphor and with a surface treatment agent, such as a silane coupling agent has been proposed in, for example, Japanese Patent Publication No. 6(1994)-31908. Also, as another example of such techniques, a technique for treating a stimulable phosphor with a titanate type of coupling agent has been proposed in, for example, Japanese Patent Publication No. 8(1996) -540363. However, for the reasons described below, the proposed techniques are not satisfactory as a technique for obtaining a radiation image storage panel having a high sensitivity by enhancing the dispersibility of the stimulable phosphor and the packing rate of the stimulable phosphor. Specifically, ordinarily, the stability of the dispersed state of the stimulable phosphor particles, which have been subjected to surface treatment with the coupling agents described above, is enhanced in the state of the coating composition. However, it often occurs that the compatibility of the stimulable phosphor particles, which have been subjected to surface treatment with the coupling agents described above, with respect to resin constituents becomes low. Therefore, it often occurs that the ultimate state of dispersion of the stimulable phosphor in the stimulable phosphor layer cannot be sufficiently enhanced.
The primary object of the present invention is to provide a radiation image storage panel, in which dispersibility of phosphor particles has been enhanced and which has a high sensitivity and yields little noise.
Another object of the present invention is to provide a process for producing a radiation image storage panel, wherein a packing rate of a phosphor in a phosphor layer is capable of being enhanced such that a decrease in sensitivity of the phosphor due to compression processing is minimized.
The present invention provides a first radiation image storage panel, comprising at least a substrate and a phosphor layer overlaid upon the substrate,
wherein the phosphor layer contains a binder, a phosphor, and at least one kind of organic phosphorus compound represented by Formula (1):
(R)nPO(OM)3xe2x88x92nxe2x80x83xe2x80x83(1)
in which R represents an alkyl group, an aryl group, or a heterocyclic group, which may be optionally substituted by a substituent;
M represents a hydrogen atom, an alkali metal, or xe2x80x94N+(R1)4, where R1 represents an alkyl group having at most two carbon atoms; and
n represents an integral number of 1 or 2.
The term xe2x80x9cradiation image storage panelxe2x80x9d as used herein means both the radiation image storage panel containing a stimulable phosphor, which radiation image storage panel is employed for radiation image recording and reproducing techniques utilizing a stimulable phosphor, and the intensifying screen utilized for the conventional radiography.
In the first radiation image storage panel in accordance with the present invention, R in Formula (1) representing the organic phosphorus compound should preferably represent an aryl group or an aryl group substituted by an alkyl group having one to four carbon atoms.
Also, in the first radiation image storage panel in accordance with the present invention, the binder should preferably be a thermoplastic elastomer having a softening temperature or a melting temperature, which falls within the range of 30xc2x0 C. to 150xc2x0 C. The binder should more preferably be a polyurethane resin.
The present invention also provides a first process for producing a radiation image storage panel, which radiation image storage panel comprises at least a substrate and a phosphor layer overlaid upon the substrate, the phosphor layer containing a binder, a phosphor, and at least one kind of organic phosphorus compound represented by Formula (1):
(R)nPO(OM)3xe2x88x92nxe2x80x83xe2x80x83(1)
in which R represents an alkyl group, an aryl group, or a heterocyclic group, which may be optionally substituted by a substituent;
M represents a hydrogen atom, an alkali metal, or xe2x80x94N+(R1)4, where R1 represents an alkyl group having at most two carbon atoms; and
n represents an integral number of 1 or 2;
the process comprising the steps of:
i) subjecting the phosphor, the organic phosphorus compound, and the binder to dispersing processing in order to prepare a coating composition, and
ii) performing application of the coating composition in order to form the phosphor layer.
The first process for producing a radiation image storage panel in accordance with the present invention should preferably be modified such that particles of the phosphor are subjected to surface treatment with the organic phosphorus compound,
the surface-treated phosphor particles and the binder are subjected to dispersing processing in order to prepare a coating composition, and
application of the coating composition is performed in order to form the phosphor layer.
Also, the first process for producing a radiation image storage panel in accordance with the present invention should preferably be modified such that the phosphor layer, which has been formed with the dispersing processing and the application of the coating composition, is located on the substrate, and
the phosphor layer having been located on the substrate is compressed at a temperature equal to at least a softening temperature or a melting temperature of the binder, the phosphor layer and the substrate being thereby adhered to each other.
(In cases where the phosphor layer is to be overlaid upon the substrate, the phosphor layer having been formed in a sheet-like shape is placed on the substrate and adhered to the substrate. Therefore, in this specification, the phosphor layer will often be referred to as a phosphor sheet.)
The present invention further provides a second radiation image storage panel, comprising at least a substrate and a phosphor layer overlaid upon the substrate,
wherein the phosphor layer contains a binder, a phosphor, and at least one kind of organic phosphorus compound represented by either one of Formula (2) and Formula (3): xe2x80x83(Rxe2x80x2xe2x80x94O)nP(OM)3xe2x88x92nxe2x80x83xe2x80x83(3)
in which Rxe2x80x2 in Formula (2) represents an aryl group or a heterocyclic group;
Rxe2x80x2 in Formula (3) represents an alkyl group, an aryl group, or a heterocyclic group, which may be optionally substituted by a substituent;
M represents a hydrogen atom, an alkali metal, or xe2x80x94N+(R1)4, where R1 represents an alkyl group having at most two carbon atoms; and
n represents an integral number of 1 or 2.
In the second radiation image storage panel in accordance with the present invention, Rxe2x80x2 in each of Formula (2) and Formula (3) representing the organic phosphorus compound should preferably represent an aryl group.
Also, in the first radiation image storage panel in accordance with the present invention, the binder should preferably be a thermoplastic elastomer having a softening temperature or a melting temperature, which falls within the range of 30xc2x0 C. to 150xc2x0 C. The binder should more preferably be a polyurethane resin.
The present invention still further provides a second process for producing a radiation image storage panel, which radiation image storage panel comprises at least a substrate and a phosphor layer overlaid upon the substrate, the phosphor layer containing a binder, a phosphor, and at least one kind of organic phosphorus compound represented by either one of Formula (2) and Formula (3): xe2x80x83(Rxe2x80x2xe2x80x94O)nP(OM)3xe2x88x92nxe2x80x83xe2x80x83(2)
in which Rxe2x80x2 in Formula (2) represents an aryl group or a heterocyclic group;
Rxe2x80x2 in Formula (3) represents an alkyl group, an aryl group, or a heterocyclic group, which may be optionally substituted by a substituent;
M represents a hydrogen atom, an alkali metal, or xe2x80x94N+(R1)4, where R1 represents an alkyl group having at most two carbon atoms; and
n represents an integral number of 1 or 2;
the process comprising the steps of:
i) subjecting the phosphor, the organic phosphorus compound, and the binder to dispersing processing in order to prepare a coating composition, and
ii) performing application of the coating composition in order to form the phosphor layer.
The second process for producing a radiation image storage panel in accordance with the present invention should preferably be modified such that particles of the phosphor are subjected to surface treatment with the organic phosphorus compound,
the surface-treated phosphor particles and the binder are subjected to dispersing processing in order to prepare a coating composition, and
application of the coating composition is performed in order to form the phosphor layer.
Also, the second process for producing a radiation image storage panel in accordance with the present invention should preferably be modified such that the phosphor layer, which has been formed with the dispersing processing and the application of the coating composition, is located on the substrate, and
the phosphor layer having been located on the substrate is compressed at a temperature equal to at least a softening temperature or a melting temperature of the binder, the phosphor layer and the substrate being thereby adhered to each other.
The present invention also provides a third radiation image storage panel, comprising at least a substrate and a phosphor layer overlaid upon the substrate,
wherein the phosphor layer contains a binder, a phosphor, and at least one kind of aralkyl phosphonic acid represented by Formula (4):
((Ar)mxe2x80x94Rxe2x80x3xe2x80x94)nPO(OMxe2x80x2)3xe2x88x92nxe2x80x83xe2x80x83(4)
in which Ar represents an aryl group;
Rxe2x80x3 represents an aliphatic group having one to six carbon atoms;
m represents an integral number of at least 1;
n represents an integral number of 1 or 2; and
Mxe2x80x2 represents a hydrogen atom, an alkali metal, or xe2x80x94N+(R3)4, where R3 represents an alkyl group having at most two carbon atoms.
In the third radiation image storage panel in accordance with the present invention, the binder should preferably be a thermoplastic elastomer having a softening temperature or a melting temperature, which falls within the range of 30xc2x0 C. to 150xc2x0 C. The binder should more preferably be a polyurethane resin.
The present invention further provides a third process for producing a radiation image storage panel, which radiation image storage panel comprises at least a substrate and a phosphor layer overlaid upon the substrate, the phosphor layer containing a binder, a phosphor, and at least one kind of aralkyl phosphonic acid represented by Formula (4):
((Ar)mxe2x80x94Rxe2x80x3xe2x80x94)nPO(OMxe2x80x2)3xe2x88x92nxe2x80x83xe2x80x83(4)
in which Ar represents an aryl group;
Rxe2x80x3 represents an aliphatic group having one to six carbon atoms;
m represents an integral number of at least 1;
n represents an integral number of 1 or 2; and
Mxe2x80x2 represents a hydrogen atom, an alkali metal, or xe2x80x94N+(R3)4, where R3 represents an alkyl group having at most two carbon atoms;
the process comprising the steps of:
i) subjecting the phosphor, the aralkyl phosphonic acid, and the binder to dispersing processing in order to prepare a coating composition, and
ii) performing application of the coating composition in order to form the phosphor layer.
The third process for producing a radiation image storage panel in accordance with the present invention should preferably be modified such that particles of the phosphor are subjected to surface treatment with the aralkyl phosphonic acid,
the surface-treated phosphor particles and the binder are subjected to dispersing processing in order to prepare a coating composition, and
application of the coating composition is performed in order to form the phosphor layer.
Also, the third process for producing a radiation image storage panel in accordance with the present invention should preferably be modified such that the phosphor layer, which has been formed with the dispersing processing and the application of the coating composition, is located on the substrate, and
the phosphor layer having been located on the substrate is compressed at a temperature equal to at least a softening temperature or a melting temperature of the binder, the phosphor layer and the substrate being thereby adhered to each other.
With the first radiation image storage panel in accordance with the present invention, the phosphor layer contains at least one kind of organic phosphorus compound represented by Formula (1):
(R)nPO(OM)3xe2x88x92nxe2x80x83xe2x80x83(1)
in which R represents an alkyl group, an aryl group, or a heterocyclic group, which may be optionally substituted by a substituent;
M represents a hydrogen atom, an alkali metal, or xe2x80x94N+(R1)4, where R1 represents an alkyl group having at most two carbon atoms; and
n represents an integral number of 1 or 2.Therefore, as described below, the advantages over the techniques, wherein a coupling agent is employed as a surface treatment agent for the phosphor particles, are capable of being obtained. Specifically, with the first radiation image storage panel in accordance with the present invention, the organic phosphorus compound is firmly adsorbed to the surfaces of the phosphor particles and is capable of forming organic monomolecular films on the surfaces of the phosphor particles. Therefore, the phosphor particles are released from agglomeration. Accordingly, the phosphor particles are capable of being uniformly dispersed in the binder, and good adhesion between the phosphor particles, which are the inorganic material, and the binder, which is the organic material, is capable of being obtained.
Also, with the first radiation image storage panel in accordance with the present invention, wherein the surfaces of the phosphor particles are treated with the organic phosphorus compound, the phosphor particles are capable of being dispersed uniformly in the phosphor layer, the affinity between the surfaces of the phosphor particles and the binder resin is capable of being enhanced, and the packing rate of the phosphor in the phosphor layer is capable of being enhanced. Therefore, the problems are capable of being prevented from occurring in that the phosphor particles break one another due to compression processing in the process for producing a radiation image storage panel.
Further, the organic phosphorus compound employed in the phosphor layer has a strong mutual action. Therefore, after the phosphor layer has been formed, the organic phosphorus compound does not undergo desorption from the surfaces of the phosphor particles. As a result, durability of the radiation image storage panel against movement operations is capable of enhanced markedly. Furthermore, since the organic phosphorus compound has a lower susceptibility to water than other organic acid compounds, such as sulfonic acid, a radiation image storage panel having a high water resistance and good durability is capable of being obtained.
With the first process for producing a radiation image storage panel in accordance with the present invention, the phosphor layer is formed by subjecting the phosphor, the organic phosphorus compound, and the binder to dispersing processing in order to prepare the coating composition, and performing the application of the coating composition. Alternatively, with the first process for producing a radiation image storage panel in accordance with the present invention, the phosphor layer may be formed by subjecting the particles of the phosphor to surface treatment with the organic phosphorus compound, subjecting the surface-treated phosphor particles and the binder to the dispersing processing in order to prepare the coating composition, and performing the application of the coating composition. Therefore, the phosphor particles are capable of being released from agglomeration, and the organic phosphorus compound is capable of being firmly adsorbed to the surfaces of the phosphor particles. Accordingly, the phosphor particles are capable of being dispersed uniformly in the phosphor layer.
With the first process for producing a radiation image storage panel in accordance with the present invention, the phosphor layer, which has been formed with the dispersing processing and the application of the coating composition, may be located on the substrate, and the phosphor layer having been located on the substrate may be compressed at a temperature equal to at least the softening temperature or the melting temperature of the binder, the phosphor layer and the substrate being thereby adhered to each other. In such cases, the phosphor is capable of being more reliably prevented from being broken.
Specifically, during the compression processing, the phosphor particles having been dispersed in the binder, which is set at the temperature equal to at least the softening temperature or the melting temperature of the binder, receive a pressure in a state in which the phosphor particles have a certain degree of movement flexibility. Therefore, under the applied pressure, the phosphor particles are capable of being orientated easily. Also, the phosphor sheet is located on the substrate without being secured to the substrate, a pressure is applied to the phosphor sheet, and the phosphor sheet is thereby adhered to the substrate while the phosphor sheet is being compressed. Therefore, even if the phosphor sheet is subjected to a pressure of a level, such that the phosphor particles will be broken if the phosphor sheet is secured to the substrate, the phosphor sheet is capable of being processed such that the phosphor particles are orientated, and the phosphor sheet is capable of being extended into a thin sheet.
With the second radiation image storage panel in accordance with the present invention, the phosphor layer contains at least one kind of organic phosphorus compound represented by either one of Formula (2) and Formula (3): xe2x80x83(Rxe2x80x2xe2x80x94O)nP(OM)3xe2x88x92nxe2x80x83xe2x80x83(3)
in which Rxe2x80x2 in Formula (2) represents an aryl group or a heterocyclic group;
Rxe2x80x2 in Formula (3) represents an alkyl group, an aryl group, or a heterocyclic group, which may be optionally substituted by a substituent;
M represents a hydrogen atom, an alkali metal, or xe2x80x94N+(R1)4, where R1 represents an alkyl group having at most two carbon atoms; and
n represents an integral number of 1 or 2. Therefore, the same effects as those with the first radiation image storage panel in accordance with the present invention are capable of being obtained.
With the second process for producing a radiation image storage panel in accordance with the present invention, the phosphor layer is formed by subjecting the phosphor, the organic phosphorus compound, and the binder to dispersing processing in order to prepare the coating composition, and performing the application of the coating composition. Alternatively, with the second process for producing a radiation image storage panel in accordance with the present invention, the phosphor layer may be formed by subjecting the particles of the phosphor to surface treatment with the organic phosphorus compound, subjecting the surface-treated phosphor particles and the binder to the dispersing processing in order to prepare the coating composition, and performing the application of the coating composition. Therefore, the phosphor particles are capable of being released from agglomeration, and the organic phosphorus compound is capable of being firmly adsorbed to the surfaces of the phosphor particles. Accordingly, the phosphor particles are capable of being dispersed uniformly in the phosphor layer.
With the second process for producing a radiation image storage panel in accordance with the present invention, as in the first process for producing a radiation image storage panel in accordance with the present invention, the phosphor layer, which has been formed with the dispersing processing and the application of the coating composition, may be located on the substrate, and the phosphor layer having been located on the substrate may be compressed at a temperature equal to at least the softening temperature or the melting temperature of the binder, the phosphor layer and the substrate being thereby adhered to each other. In such cases, the phosphor is capable of being more reliably prevented from being broken.
With the third radiation image storage panel in accordance with the present invention, the phosphor layer contains at least one kind of aralkyl phosphonic acid represented by Formula (4):
((Ar)mxe2x80x94Rxe2x80x3xe2x80x94)nPO(OMxe2x80x2)3xe2x88x92nxe2x80x83xe2x80x83(4)
in which Ar represents an aryl group;
Rxe2x80x3 represents an aliphatic group having one to six carbon atoms;
m represents an integral number of at least 1;
n represents an integral number of 1 or 2; and
Mxe2x80x2 represents a hydrogen atom, an alkali metal, or xe2x80x94N+(R3)4, where R3 represents an alkyl group having at most two carbon atoms.
Therefore, as described below, the advantages over the techniques, wherein a coupling agent is employed as a surface treatment agent for the phosphor particles, are capable of being obtained. Specifically, with the third radiation image storage panel in accordance with the present invention, the aralkyl phosphonic acid is firmly adsorbed to the surfaces of the phosphor particles and is capable of forming organic monomolecular films on the surfaces of the phosphor particles. Therefore, the phosphor particles are released from agglomeration. Accordingly, the phosphor particles are capable of being uniformly dispersed in the binder, and good adhesion between the phosphor particles, which are the inorganic material, and the binder, which is the organic material, is capable of being obtained.
Also, with the third radiation image storage panel in accordance with the present invention, wherein the surfaces of the phosphor particles are treated with the aralkyl phosphonic acid, the phosphor particles are capable of being dispersed uniformly in the phosphor layer, the affinity between the surfaces of the phosphor particles and the binder resin is capable of being enhanced, and the packing rate of the phosphor in the phosphor layer is capable of being enhanced. Therefore, the problems are capable of being prevented from occurring in that the phosphor particles break one another due to compression processing in the process for producing a radiation image storage panel.
Further, the aralkyl phosphonic acid employed in the phosphor layer has a strong mutual action. Therefore, after the phosphor layer has been formed, the organic phosphorus compound does not undergo desorption from the surfaces of the phosphor particles. As a result, durability of the radiation image storage panel against movement operations is capable of enhanced markedly. Furthermore, since the aralkyl phosphonic acid has a lower susceptibility to water than other organic acid compounds, such as sulfonic acid, a radiation image storage panel having a high water resistance and good durability is capable of being obtained.
With the third process for producing a radiation image storage panel in accordance with the present invention, the phosphor layer is formed by subjecting the phosphor, the aralkyl phosphonic acid, and the binder to dispersing processing in order to prepare the coating composition, and performing the application of the coating composition. Alternatively, with the third process for producing a radiation image storage panel in accordance with the present invention, the phosphor layer may be formed by subjecting the particles of the phosphor to surface treatment with the aralkyl phosphonic acid, subjecting the surface-treated phosphor particles and the binder to the dispersing processing in order to prepare the coating composition, and performing the application of the coating composition. Therefore, the phosphor particles are capable of being released from agglomeration, and the aralkyl phosphonic acid is capable of being firmly adsorbed to the surfaces of the phosphor particles. Accordingly, the phosphor particles are capable of being dispersed uniformly in the phosphor layer.
With the third process for producing a radiation image storage panel in accordance with the present invention, as in the first and second processes for producing a radiation image storage panel in accordance with the present invention, the phosphor layer, which has been formed with the dispersing processing and the application of the coating composition, may be located on the substrate, and the phosphor layer having been located on the substrate may be compressed at a temperature equal to at least the softening temperature or the melting temperature of the binder, the phosphor layer and the substrate being thereby adhered to each other. In such cases, the phosphor is capable of being more reliably prevented from being broken.
The radiation image storage panels in accordance with the present invention will hereinbelow be described in detail. Firstly, phosphors capable of being employed in the first, second, and third radiation image storage panels in accordance with the present invention will be described hereinbelow.
As the phosphor, a stimulable phosphor may be employed. The stimulable phosphor has the properties such that, when the stimulable phosphor is caused to absorb radiation and is then exposed to stimulating rays, the stimulable phosphor emits light in proportion to the amount of energy stored thereon during its exposure to the radiation. From the practical aspect, the stimulable phosphor should preferably have the characteristics such that, when the stimulable phosphor is exposed to the stimulating rays having wavelengths falling within the range of 400 nm to 900 nm, the stimulable phosphor emits light having wavelengths falling within the range of 300 nm to 500 nm. Examples of the stimulable phosphors, which may be employed in the radiation image storage panels in accordance with the present invention, include the following:
a phosphor represented by the formula BaSO4:AX, as described in Japanese Unexamined Patent Publication No. 48 (1973)-80487,
a phosphor represented by the formula SrSO4:AX, as described in Japanese Unexamined Patent Publication No. 48 (1973)-80489,
a phosphor represented by the formula Li2B4O7:Cu ,Ag, as described in Japanese Unexamined Patent Publication No. 53(1978)-39277,
a phosphor represented by the formula Li2Oxe2x80x2(B2O2)x:Cu; or Li2Oxe2x80x2(B2O2)x:Cu,Ag, as described in Japanese Unexamined Patent Publication No. 54(1979)-47883,
a phosphor represented by the formula SrS:Ce,Sm; SrS:Eu,Sm; ThO2:Er; or La2O2S:Eu,Sm, as described in U.S. Pat. No. 3,859,527,
a phosphor represented by the formula ZnS:Cu,Pb; BaOxe2x80x2xAl2O3:Eu wherein 0.8xe2x89xa6xxe2x89xa610; MIIOxe2x80x2xSiO2:A wherein MII is Mg, Ca, Sr, Zn, Cd, or Ba, A is Ce, Tb, Eu, Tm, Pb, Tl, Bi, or Mn, and x is a number satisfying 0.5xe2x89xa6xxe2x89xa62.5; or LnOX:xA wherein Ln is at least one of La, Y, Gd, and Lu, X is at least one of Cl and Br, A is at least one of Ce and Tb, x is a number satisfying 0 less than x less than 0.1, as disclosed in U.S. Pat. No. 4,236,078,
a phosphor represented by the formula (Ba1xe2x88x92xxe2x88x92y, Mgx, Cay)FX:aEu2+ wherein X is at least one of Cl and Br, x and y are numbers satisfying 0 less than x+yxe2x89xa60.6 and xyxe2x89xa00, and a is a number satisfying 10xe2x88x926xe2x89xa6axe2x89xa65xc3x9710xe2x88x922, as disclosed in DE-OS No. 2,928,245,
a phosphor represented by the formula (Ba1xe2x88x92x, M2+x)FX:yA wherein M2+ is at least one of Mg, Ca, Sr, Zn, and Cd, X is at least one of Cl, Br, and I, A is at least one of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb, and Er, x is a number satisfying 0xe2x89xa6xxe2x89xa60.6, and y is a number satisfying 0xe2x89xa6yxe2x89xa60.2, as disclosed in U.S. Pat. No. 4,239,968,
a phosphor represented by the formula BaFX:xCexe2x80x2yA, as described in Japanese Unexamined Patent Publication No. 55(1980)-843897,
a phosphor represented by the formula MIIFXxe2x80x2xA:yLn wherein MII is at least one of Ba, Ca, Sr, Mg, Zn, and Cd, A is at least one of BeO, MgO, CaO, SrO, BaO, ZnO, Al2O3, Y2O3, La2O3, In2O3, SiO2, TiO2, ZrO2, GeO2, SnO2, Nb2O5, Ta2O5, and ThO2, Ln is at least one of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb, Er, Sm, and Gd, X is at least one of Cl, Br, and I, x is a number satisfying 5xc3x9710xe2x88x925xe2x89xa6xxe2x89xa60.5, and y is a number satisfying 0 less than yxe2x89xa60.2, as described in Japanese Unexamined Patent Publication No. 55(1980)-160078,
a phosphor represented by the formula (Ba1xe2x88x92x, MIIx)F2xe2x80x2aBaX2:yEu, zA wherein MII is at least one of beryllium, magnesium, calcium, strontium, zinc, and cadmium, X is at least one of chlorine, bromine, and iodine, A is at least one of zirconium and scandium, a is a number satisfying 0.5xe2x89xa6axe2x89xa61.25, x is a number satisfying 0xe2x89xa6xxe2x89xa61, y is a number satisfying 10xe2x88x926xe2x89xa6yxe2x89xa62xc3x9710xe2x88x921, and z is a number satisfying 0 less than zxe2x89xa610xe2x88x922, as described in Japanese Unexamined Patent Publication No. 56(1981)-116777,
a phosphor represented by the formula (Ba1xe2x88x92x,MIIx)F2xc2x7aBaX2:yEu, zB wherein MII is at least one of beryllium, magnesium, calcium, strontium, zinc, and cadmium, X is at least one of chlorine, bromine, and iodine, a is a number satisfying 0.5xe2x89xa6axe2x89xa61.25, x is a number satisfying 0xe2x89xa6xxe2x89xa61, y is a number satisfying 10xe2x88x926xe2x89xa6yxe2x89xa62xc3x9710xe2x88x921, and z is a number satisfying 0 less than zxe2x89xa610xe2x88x922, as described in Japanese Unexamined Patent Publication No. 57(1982)-23673,
a phosphor represented by the formula (Ba1xe2x88x92x, MIIx)F2xc2x7aBaX2:yEu, zA wherein MII is at least one of beryllium, magnesium, calcium, strontium, zinc, and cadmium, X is at least one of chlorine, bromine, and iodine, A is at least one of arsenic and silicon, a is a number satisfying 0.5 xe2x89xa6axe2x89xa61.25, x is a number satisfying 0xe2x89xa6xxe2x89xa61, y is a number satisfying 10xe2x88x926xe2x89xa6yxe2x89xa62xc3x9710xe2x88x921, and z is a number satisfying 0 less than zxe2x89xa65xc3x9710xe2x88x921, as described in Japanese Unexamined Patent Publication No. 57(1982)-23675,
a phosphor represented by the formula MIIIOX:xCe wherein MIII is at least one trivalent metal selected from the group consisting of Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, and Bi, X is either one or both of Cl and Br, and x is a number satisfying 0 less than x less than 0.1, as described in Japanese Unexamined Patent Publication No. 58(1983)-69281,
a phosphor represented by the formula Ba1xe2x88x92xMx/2Lx/2FX:yEu2+ wherein M is at least one alkaline metal selected from the group consisting of Li, Na, K, Rb, and Cs, L is at least one trivalent metal selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Ga, In, and Tl, X is at least one halogen selected from the group consisting of Cl, Br, and I, x is a number satisfying 10xe2x88x922xe2x89xa6xxe2x89xa60.5, and y is a number satisfying 0 less than yxe2x89xa60.1, as described in Japanese Unexamined Patent Publication No. 58(1983)-206678,
a phosphor represented by the formula BaFXxc2x7xA:yEu2+, wherein X is at least one halogen selected from the group consisting of Cl, Br, and I, A is a calcination product of a tetrafluoro boric acid compound, x is a number satisfying 10xe2x88x926xe2x89xa6xxe2x89xa60.1, and y is a number satisfying 0 less than yxe2x89xa60.1, as described in Japanese Unexamined Patent Publication No. 59(1984)-27980,
a phosphor represented by the formula xM3(PO4)2xc2x7NX2:yA; M3(PO4)2:yA; nReX3xc2x7mAXxe2x80x22:xEu; nReX3xc2x7mAXxe2x80x22:xEu, ySm; or MIXxc2x7aMIIXxe2x80x22xc2x7bMIIIXxe2x80x33:cA, as described in Japanese Unexamined Patent Publication No. 59(1984)-38278,
a phosphor represented by the formula BaFXxc2x7xA:yEu2+ wherein X is at least one halogen selected from the group consisting of Cl, Br, and I, A is a calcination product of at least one compound selected from the hexafluoro compound group consisting of salts of hexafluoro silicic acid, hexafluoro titanic acid, and hexafluoro zirconic acid with monovalent or bivalent metals, x is a number satisfying 10xe2x88x926xe2x89xa6xxe2x89xa60. 1 and y is a number satisfying 0 less than yxe2x89xa60.1, as described in Japanese Unexamined Patent Publication No. 59(1984) -47289,
a phosphor represented by the formula BaFXxc2x7xNaXxe2x80x2:aEu2+ wherein each of X and Xxe2x80x2 is at least one of Cl, Br, and I, x is a number satisfying 0 less than xxe2x89xa62, and a is a number satisfying 0 less than axe2x89xa60.2, as described in Japanese Unexamined Patent Publication No. 59(1984)-56479,
a phosphor represented by the formula MIIFXxc2x7xNaXxe2x80x2:yEu2+:zA wherein MII is at least one alkaline earth metal selected from the group consisting of Ba, Sr, and Ca, each of X and Xxe2x80x2 is at least one halogen selected from the group consisting of Cl, Br, and I, A is at least one transition metal selected from the group consisting of V, Cr, Mn, Fe, Co, and Ni, x is a number satisfying 0 less than xxe2x89xa62, y is a number satisfying 0 less than yxe2x89xa60.2, and z is a number satisfying 0 less than zxe2x89xa610xe2x88x922, as described in Japanese Unexamined Patent Publication No. 59(1984)-56480,
a phosphor represented by the formula MIIFXxc2x7aMIXxe2x80x2xc2x7bMxe2x80x2IIXxe2x80x32xc2x7cMIIIXxe2x80x3xe2x80x23xc2x7xA: yEu2+ wherein MII is at least one alkaline earth metal selected from the group consisting of Ba, Sr, and Ca, MI is at least one alkali metal selected from the group consisting of Li, Na, K, Rb, and Cs, Mxe2x80x2II is at least one bivalent metal selected from the group consisting of Be and Mg, MIII is at least one trivalent metal selected from the group consisting of Al, Ga, In, and Tl, A is a metal oxide, X is at least one halogen selected from the group consisting of Cl, Br, and I, each of Xxe2x80x2, Xxe2x80x3, and Xxe2x80x3xe2x80x2 is at least one halogen selected from the group consisting of F, Cl, Br, and I, a is a number satisfying 0xe2x89xa6axe2x89xa62, b is a number satisfying 0xe2x89xa6bxe2x89xa610xe2x88x922, c is a number satisfying 0xe2x89xa6cxe2x89xa610xe2x88x922, and a+b+cxe2x89xa710xe2x88x926, x is a number satisfying 0 less than xxe2x89xa60.5, and y is a number satisfying 0 less than yxe2x89xa60.2, as described in Japanese Unexamined Patent Publication No. 59(1984)-75200,
a stimulable phosphor represented by the formula MIIX2xc2x7aMIIXxe2x80x22:xEu2+ wherein MII is at least one alkaline earth metal selected from the group consisting of Ba, Sr, and Ca, each of X and Xxe2x80x2 is at least one halogen selected from the group consisting of Cl, Br, and I, and Xxe2x89xa0Xxe2x80x2, a is a number satisfying 0.1xe2x89xa6axe2x89xa610.0, and x is a number satisfying 0 less than xxe2x89xa60.2, as described in Japanese Unexamined Patent Publication No. 60(1985)-84381,
a stimulable phosphor represented by the formula MIIFXxc2x7aMIXxe2x80x2:xEu2+ wherein MII is at least one alkaline earth metal selected from the group consisting of Ba, Sr, and Ca, MI is at least one alkali metal selected from the group consisting of Rb and Cs, X is at least one halogen selected from the group consisting of Cl, Br, and I, Xxe2x80x2 is at least one halogen selected from the group consisting of F, Cl, Br, and I, a is a number satisfying 0xe2x89xa6axe2x89xa64.0, and x is a number satisfying 0 less than xxe2x89xa60.2, as described in Japanese Unexamined Patent Publication No. 60(1985)-101173, and
a stimulable phosphor represented by the formula MIX:xBi wherein MI is at least one alkali metal selected from the group consisting of Rb and cs, X is at least one halogen selected from the group consisting of Cl, Br, and I, and x is a number falling within the range of 0 less than xxe2x89xa60.2, as described in Japanese Unexamined Patent Publication No. 62(1987)-25189.
The stimulable phosphor represented by the formula MIIX2xc2x7aMIIXxe2x80x22:xEu2+, which is described in Japanese Unexamined Patent Publication No. 60(1985)-84381, may contain the additives described below in the below-mentioned proportions per mol of MIIX2xc2x7aMIIXxe2x80x22:
bMIXxe2x80x3 wherein MI is at least one alkali metal selected from the group consisting of Rb and Cs, Xxe2x80x3 is at least one halogen selected from the group consisting of F, Cl, Br, and I, and b is a number satisfying 0 less than bxe2x89xa610.0, as described in Japanese Unexamined Patent Publication No. 60(1985) -166379,
bKXxe2x80x3xc2x7cMgX2xc2x7dMIIIXxe2x80x23 wherein MIII is at least one trivalent metal selected from the group consisting of Sc, Y, La, Gd, and Lu, each of Xxe2x80x3, X, and Xxe2x80x2 is at least one halogen selected from the group consisting of F, Cl, Br, and I, b is a number satisfying 0xe2x89xa6bxe2x89xa62.0, c is a number satisfying 0xe2x89xa6cxe2x89xa62.0, d is a number satisfying 0xe2x89xa6dxe2x89xa62.0, and 2xc3x9710xe2x88x925xe2x89xa6b+c+d, as described in Japanese Unexamined Patent Publication No. 60(1985) -221483,
yB wherein y is a number satisfying 2xc3x9710xe2x88x924xe2x89xa6yxe2x89xa62xc3x9710xe2x88x921, as described in Japanese Unexamined Patent Publication No. 60(1985)-228592,
bA wherein A is at least one oxide selected from the group consisting of SiO2 and P2O5, and b is a number satisfying 10xe2x88x924xe2x89xa6bxe2x89xa62xc3x9710xe2x88x921, as described in Japanese Unexamined Patent Publication No. 60(1985)-228593,
bSiO wherein b is a number satisfying 0 less than bxe2x89xa63xc3x9710xe2x88x922 as described in Japanese Unexamined Patent Publication No. 61(1986)-120883,
bSnXxe2x80x32 wherein Xxe2x80x3 is at least one halogen selected from the group consisting of F, Cl, Br, and I, and b is a number satisfying 0 less than bxe2x89xa610xe2x88x923, as described in Japanese Unexamined Patent Publication No. 61(1986)-120885,
bCsXxe2x80x3xc2x7cSnX2 wherein each of Xxe2x80x3 and X is at least one halogen selected from the group consisting of F, Cl, Br, and I, b is a number satisfying 0 less than bxe2x89xa610.0, and c is a number satisfying 10xe2x88x926xe2x89xa6cxe2x89xa62xc3x9710xe2x88x922, as described in Japanese Unexamined Patent Publication No. 61(1986)-235486, and
bCsXxe2x80x3xc2x7yLn3+ wherein Xxe2x80x3 is at least one halogen selected from the group consisting of F, Cl, Br, and I, Ln is at least one rare earth element selected from the group consisting of Sc, Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, b is a number satisfying 0 less than bxe2x89xa610.0, and y is a number satisfying 10xe2x88x926xe2x89xa6yxe2x89xa61.8xc3x9710xe2x88x921, as described in Japanese Unexamined Patent Publication No. 61(1986)-235487.
Further, it is possible to employ a rare earth element-activated alkaline earth metal fluorohalide type of stimulable phosphor represented by Formula (I) acting as a fundamental composition formula:
(Ba1xe2x88x92a, MIIa) FX:zLnxe2x80x83xe2x80x83(I)
in which MII is at least one alkaline earth metal selected from the group consisting of Sr and Ca, Ln is at least one rare earth element selected from the group consisting of Ce, Pr, Sm, Eu, Tb, Dy, Ho, Nd, Er, Tm, and Yb, X is at least one halogen selected from the group consisting of Cl, Br, and I, a is a number satisfying 0xe2x89xa6a less than 1, and z is a number satisfying 0xe2x89xa6z less than 0.2. In Formula (I), a should preferably be a number of at most 0.5. Also, Ln should preferably be Eu or Ce. Formula (I) does not represent that the ratio F:X is stoichiometrically such that F:X=1:1, and represents that the composition is a compound having a PbFCl type of crystal structure represented by (Ba1xe2x88x92a, MIIa)FX. In general, a state of a BaFX crystal, in which a large number of F+(Xxe2x88x92) center points, which are the vacancies of X-ions, have been formed, is preferable for enhancing the light emission efficiency with respect to light having wavelengths falling within the range of 600 nm to 700 nm. In such cases, F will be in slight excess of X.
Though not defined in Formula (I) , when necessary, additives described below may be added to the stimulable phosphor represented by Formula (I):
bA, wNI, xNII, yNIII
In the additives, NI represents at least one alkali metal compound selected from the group consisting of compounds of Li, Na, K, Rb, and Cs, NII represents at least one alkaline earth metal compound selected from the group consisting of compounds of Mg and Be, and NIII represents at least one trivalent metal compound selected from the group consisting of compounds of Al, Ga, In, Tl, Sc, Y, La, Gd, and Lu. As the metal compounds, halides as described in Japanese Unexamined Patent Publication No. 59(1984)-75200 should preferably be employed. However, the metal compounds are not limited to the halides. A represents a metal oxide, such as Al2O3, SiO2, or ZrO2. In order for BaFx particles to be prevented from being sintered together, the metal oxide should preferably take on the form of ultrafine particles such that the mean particle diameter of primary particles is at most 0.1 xcexcm, and the metal oxide should preferably have a low reactivity with respect to (Ba1xe2x88x92a, MIIa)FX. In particular, the metal oxide should preferably be Al2O3. Also, b, w, x, and y represent the amounts of the compounds added per mol of (Ba1xe2x88x92a, MIIa) FX and are numbers respectively satisfying 0xe2x89xa6bxe2x89xa60.5, 0xe2x89xa6wxe2x89xa62, 0xe2x89xa6xxe2x89xa60.3, and 0xe2x89xa6yxe2x89xa60.3. As for an additive, whose weight decreases due to calcination and subsequent washing processing, the number representing the amount of the compound added does not represent the element ratio in a finally obtained composition. Certain additives will remain as the added compounds in the finally obtained composition, and certain additives will react with BaFX or will be taken into BaFX.
Further, when necessary, compounds described below may be employed as the additives:
Zn and Cd compounds, as described in Japanese Unexamined Patent Publication No. 55(1980)-12145,
metal oxides TiO2, BeO, MgO, CaO, SrO, BaO, ZnO, Y2O3, La2O3, In2O3, GeO2, SnO2, Nb2O5, Ta2O5, and ThO2, as described in Japanese Unexamined Patent Publication No. 55(1980) -160078,
Zr and Sc compounds, as described in Japanese Unexamined Patent Publication No. 56(1981)-116777,
B compounds, as described in Japanese Unexamined Patent Publication No. 57(1982)-23673,
As and Si compounds, as described in Japanese Unexamined Patent Publication No. 57(1982)-23675,
tetrafluoro boric acid compounds, as described in Japanese Unexamined Patent Publication No. 59(1984)-27980,
hexafluoro compounds constituted of monovalent or bivalent salts of hexafluoro silicic acid, hexafluoro titanic acid, or hexafluoro zirconic acid, as described in Japanese Unexamined Patent Publication No. 59(1984)-47289, and
compounds of transition metals, such as V, Cr, Mn, Fe, Co, and Ni, as described in Japanese Unexamined Patent Publication No. 59(1984)-56480.
However, the rare earth element-activated alkaline earthmetal fluorohalide type of stimulable phosphor, which may be employed in the radiation image storage panels in accordance with the present invention, is not limited to the phosphors containing the additives described above and may be selected from various stimulable phosphors, which basically contain the composition regarded as the rare earth element-activated alkaline earthmetal fluorohalide type of stimulable phosphor.
Ordinarily, the rare earth element-activated alkaline earth metal fluorohalide type of stimulable phosphor represented by Formula (I) acting as the fundamental composition formula have a particle aspect ratio falling within the range of 1.0 to 5.0. The rare earth element-activated alkaline earth metal fluorohalide type of stimulable phosphor, which may be employed in the radiation image storage panels in accordance with the present invention, should preferably have a particle aspect ratio falling within the range of 1.0 to 2.0 (and should more preferably have a particle aspect ratio falling within the range of 1.0 to 1.5), should preferably have a particle size median diameter (Dm) falling within the range of 1 xcexcm to 10 xcexcm (and should more preferably have a particle size median diameter (Dm) falling within the range of 2 xcexcm to 7 xcexcm), and should preferably have a value "sgr"/Dm (where "sgr" represents the standard deviation of the particle size distribution) of at most 50% (and should more preferably have a value "sgr"/Dm of at most 40%). The shape of the particles may be a rectangular parallelepiped form, a regular hexahedral form, a regular octahedral form, one of intermediate polyhedral forms, a tetradecahedral form, or the like. The shape of the particles should preferably be a tetradecahedral form. However, the shape of the particles may be one of various shapes other than the tetradecahedral form, which satisfy the aforesaid requirements with respect to the particle aspect ratio, the particle size, and the particle size distribution.
Of the above-enumerated stimulable phosphors, the bivalent europium activated alkaline earth metal halide phosphor and the cerium activated rare earth element oxyhalide phosphor exhibit light emission with a high luminance and therefore are particularly preferable. However, the stimulable phosphor employed in the radiation image storage panels in accordance with the present invention is not limited to the phosphors enumerated above and may be one of various other phosphors, which have the properties such that, when the phosphors are caused to absorb radiation and are then exposed to stimulating rays, the phosphors emit light in proportion to the amount of energy stored thereon during exposure to the radiation.
Of the phosphors enumerated above, the phosphor particles, whose surfaces have been partially oxidized through calcination or cooling in a weakly oxidizing atmosphere at an intermediate stage or a final stage of a calcination processing, exhibit good erasing characteristics and therefore are particularly preferable.
In cases where the first, second, and third radiation image storage panels in accordance with the present invention are utilized as the intensifying screens, the phosphors listed below may be employed appropriately:
a tungstate type of phosphor, such as caWo4, MgWO4, or CaWO4:Pb,
a terbium activated rare earth element acid sulfide type of phosphor, such as Y2O2S:Tb, Gd2O2S:Tb, La2O2S:Tb, (Y,Gd)2O2S:Tb, or (Y,Gd)O2S:Tb,Tm,
a terbium activated rare earth element phosphate type of phosphor, such as YPO4:Tb, GdPO4:Tb, or LaPO4:Tb,
a terbium activated rare earth element oxyhalide type of phosphor, such as LaOBr:Tb, LaOBr:Tb,Tm, LaOCl:Tb, LaOCl:Tb,Tm, GdOBr:Tb, or GdOCl:Tb,
a thulium activated rare earth element oxyhalide type of phosphor, such as LaOBr:Tm or LaOCl:Tm,
a barium sulfate type of phosphor, such as BaSO4:Pb, BaSO4:Eu2+, or (Ba, Sr)SO4:Eu2+,
a bivalent europium activated alkaline earth metal phosphate type of phosphor, such as Ba3(PO4)2:Eu2+,
a bivalent europium activated alkaline earth metal fluorohalide type of phosphor, such as BaFCl:Eu2+, BaFBr:Eu2+, BaFCl:Eu2+, Tb, BaFBr:Eu2+, Tb, BaF2xc2x7BaCl2xc2x7KCl:Eu2+, or (Ba, Mg)F2xc2x7BaCl2xc2x7KCl:Eu2+,
an iodide type of phosphor, such as CsI:Na, CsI:Tl, NaI, or KI:Tl,
a sulfide type of phosphor, such as ZnS:Ag, (Zn,Cd)S:Ag, (Zn, Cd)S:Cu, or (Zn,Cd)S:Cu,Al,
a hafnium phosphate type of phosphor, such as HfP2O7:Cu, and
a tantalate type of phosphor, such as YTaO4, YTaO4:Tm, YTaO4:Nb, (Y, Sr)TaO4xe2x88x92x:Nb, LuTaO4, LuTaO4:Nb, (Lu,Sr)TaO4xe2x88x92x:Nb, GdTaO4:Tm, or Gd2O3xc2x7Ta2O5xc2x7B2O3:Tb.
However, the phosphor employed in the radiation image storage panels in accordance with the present invention is not limited to the phosphors enumerated above and may be one of various other phosphors, which have the properties such that, when the phosphors are exposed to radiation, the phosphors emit light of the visible region or the near ultraviolet region.
The organic phosphorus compound, which may be employed in the first radiation image storage panel in accordance with the present invention, is represented by Formula (1):
(R)nPO(OM)3xe2x88x92nxe2x80x83xe2x80x83(1)
in which R represents an alkyl group, an aryl group, or a heterocyclic group, which may be optionally substituted by a substituent;
M represents a hydrogen atom, an alkali metal, or xe2x80x94N+(R1)4, where R1 represents an alkyl group having at most two carbon atoms; and
n represents an integral number of 1 or 2.
Examples of M include a hydrogen atom, sodium, potassium, and a tetraethylammonium ion.
R should preferably be an aryl group or an aryl group substituted by methyl or ethyl. Preferable examples of R include alkyl groups having one to four carbon atoms, which may optionally be substituted by an aryl group, such as a methyl group, an ethyl group, a propyl group, a butyl group, an isopropyl group, and a diphenylmethyl group; and aryl groups, which may optionally be substituted by methyl or ethyl, such as a phenyl group, a naphthyl group, an anthryl group, a diphenyl group, a p-ethylphenyl group, a tolyl group, and a xylyl group. The aryl groups may contain a ring, such as an indene ring or a tetralin ring, other than the benzene ring.
The organic phosphorus compound described above may be used alone, or two or more organic phosphorus compounds may be used in combination. Among the organic phosphorus compounds described above, organic phosphorus compounds having an aryl group are preferable, and organic phosphorus compounds having a phenyl group are more preferable.
Preferable examples of the organic phosphorus compounds, which may be employed in the first radiation image storage panel in accordance with the present invention, include the compounds described below.
(1) p-Ethylbenzene Phosphonic Acid 
(2) Phenyl Phosphonic Acid 
(3) Salts of the Phosphonic Acids Shown Above
(4) Diphenyl Phosphinic Acid 
(5) Salts of the Phosphinic Acid Shown Above
The organic phosphorus compound, which may be employed in the second radiation image storage panel in accordance with the present invention, is represented by either one of Formula (2) and Formula (3): xe2x80x83(Rxe2x80x2xe2x80x94O)nP(OM)3xe2x88x92nxe2x80x83xe2x80x83(3)
in which Rxe2x80x2 in Formula (2) represents an aryl group or a heterocyclic group;
Rxe2x80x2 in Formula (3) represents an alkyl group, an aryl group, or a heterocyclic group, which may be optionally substituted by a substituent;
M represents a hydrogen atom, an alkali metal, or xe2x80x94N+(R1)4, where R1 represents an alkyl group having at most two carbon atoms; and
n represents an integral number of 1 or 2.
Rxe2x80x2 should preferably be an aryl group or an aryl group substituted by methyl or ethyl.
Preferable examples of Rxe2x80x2 include alkyl groups having one to four carbon atoms, which may optionally be substituted by an aryl group, such as a methyl group, an ethyl group, a propyl group, a butyl group, an isopropyl group, and a diphenylmethyl group; and aryl groups, which may optionally be substituted by methyl or ethyl, such as a phenyl group, a naphthyl group, an anthryl group, a diphenyl group, a p-ethylphenyl group, a tolyl group, and a xylyl group. The aryl groups may contain a ring, such as an indene ring or a tetralin ring, other than the benzene ring.
Preferable examples of the organic phosphorus compounds represented by Formula (2) or Formula (3), which may be employed in the second radiation image storage panel in accordance with the present invention, include the compounds described below.
(1) xcex1-Naphthyl Phosphoric Acid 
(2) Phenyl Phosphoric Acid 
(3) Diphenyl Phosphoric Acid 
(4) Salts of Mono- or Di-esters of Phosphoric Acids Shown Above
(5) Di-n-butyl Phosphite
(nxe2x88x92C4H9O)2P(OH)
(6) Diethyl Phosphite
(nxe2x88x92C2H5O)2P(OH)
(7) Diphenyl Phosphite 
(8) Salts of Mono- or Di-esters of the Phosphites Shown Above
Among the organic phosphorus compounds described above, which are represented by Formula (2) or Formula (3) organic phosphorus compounds having an aryl group are preferable, and organic phosphorus compounds having a phenyl group are more preferable.
There is no risk that colors of the organic phosphorus compounds contained in the first and second radiation image storage panels in accordance with the present invention will affect adversely upon the stimulating rays or light emitted by the radiation image storage panels. Therefore, the organic phosphorus compounds should preferably be colorless or should preferably have a light yellow color.
Ordinarily, the organic phosphorus compounds have the properties such that the organic phosphorus compounds are adsorbed or bonded to metal surfaces via the polar groups described above. It is presumed that, in the phosphor layers of the first and second radiation image storage panels in accordance with the present invention, the organic phosphorus compounds are present in the state, in which the organic phosphorus compounds are adsorbed or bonded primarily to the surfaces of the phosphor particles via the polar groups described above. The organic phosphorus compounds have the advantages over organic acid compounds, such as carboxylic acids and sulfonic acids, in that the adsorption force with respect to the metal surfaces is strong, and the organic phosphorus compounds are not apt to undergo desorption from the metal surfaces. Therefore, it is presumed that the organic phosphorus compounds are firmly adsorbed to the surfaces of the phosphor particles contained in the phosphor layers of the first and second radiation image storage panels in accordance with the present invention, and the surfaces of the phosphor particles are brought into the state such that the surfaces are covered with the aromatic rings, and the like. As a result, the affinity of the phosphor particles with respect to the resin constituents will presumably be enhanced, and the stability of the dispersed state of the phosphor particles will presumably be enhanced.
Also, it is considered that, by virtue of the effects of the organic phosphorus compound described above, the phosphor particles and the binder have a strong mutual action. Therefore, it is considered that, after the phosphor layer has been formed, the binder does not undergo desorption from the surfaces of the phosphor particles. As a result, durability of the radiation image storage panel against movement operations is capable of enhanced markedly. Furthermore, since the organic phosphorus compounds employed in the first and second radiation image storage panels in accordance with the present invention have a lower susceptibility to water than other organic acid compounds, such as sulfonic acid, a radiation image storage panel having a high water resistance and good durability is capable of being obtained.
In each of the first and second radiation image storage panels in accordance with the present invention, ordinarily, the content of the organic phosphorus compound in the phosphor layer should preferably fall within the range of 0.001 to 5 parts by weight per 100 parts by weight of the phosphor particles. Such that a better dispersed state of the phosphor particles may be obtained and the glossiness of the phosphor layer surface may be enhanced, the content of the organic phosphorus compound in the phosphor layer should more preferably fall within the range of 0.005 to 2 parts by weight per 100 parts by weight of the phosphor particles. Also, such that the light emission characteristics may be enhanced markedly, the content of the organic phosphorus compound in the phosphor layer should most preferably fall within the range of 0.01 to 1 part by weight per 100 parts by weight of the phosphor particles. If the content of the organic phosphorus compound in the phosphor layer is lower than 0.001 part by weight per 100 parts by weight of the phosphor particles, it will often become impossible to obtain the effects of the organic phosphorus compound. Even if the content of the organic phosphorus compound in the phosphor layer is higher than 5 parts by weight per 100 parts by weight of the phosphor particles, further enhancement of the dispersed state of the phosphor particles cannot be expected.
In the third radiation image storage panel in accordance with the present invention, the phosphor layer contains the stimulable phosphor or the phosphor described above, the binder, and at least one kind of aralkyl phosphonic acid represented by Formula (4):
xe2x80x83((Ar)mxe2x80x94Rxe2x80x3xe2x80x94)nPO(OMxe2x80x2)3xe2x88x92nxe2x80x83xe2x80x83(4)
in which Ar represents an aryl group;
Rxe2x80x3 represents an aliphatic group having one to six carbon atoms;
m represents an integral number of at least 1;
n represents an integral number of 1 or 2; and
Mxe2x80x2 represents a hydrogen atom, an alkali metal, or xe2x80x94N+(R3)4, where R3 represents an alkyl group having at most two carbon atoms.
In the third radiation image storage panel in accordance with the present invention, the aralkyl phosphonic acid should preferably be represented by Formula (4), in which Ar represents a phenyl group or a substituted phenyl group, Rxe2x80x3 represents analiphatic group having one or two carbon atoms, and m represents an integral number of 1 or 2.
The aralkyl phosphonic acid acting as the surface modifying agent must contain at least one kind of aromatic ring and a phosphonic acid group. As clear from Formula (4), the phosphonic acid group is separated by at least one carbon atom from the aromatic ring. Examples of the aralkyl phosphonic acids, which may be employed preferably in the third radiation image storage panel in accordance with the present invention, include phenethyl phosphonic acid; benzhydryl phosphonic acid; benzyl phosphonic acid; and substitution derivatives of the above enumerated phosphonic acids, such as 4-nitrobenzyl phosphonic acid, 3-methoxybenzyl phosphonic acid, 4-methoxybenzyl phosphonic acid, 4-t-butylbenzyl phosphonic acid, 4-methylthiobenzyl phosphonic acid, 4-methanesulfonylbenzyl phosphonic acid, 4-trifluoromethylbenzyl phosphonic acid, 2,3-difluorobenzyl phosphonic acid, and 2,3,4,5,6-pentafluorobenzyl phosphonic acid. Synthesis of the above-enumerated aralkyl phosphonic acids is described in detail in Japanese Unexamined Patent Publication No. 10(1998)-70022.
The aralkyl phosphonic acids have the properties such that the aralkyl phosphonic acids are adsorbed or bonded to metal surfaces via the polar groups described above. It is presumed that, in the phosphor layer of the third radiation image storage panel in accordance with the present invention, the aralkyl phosphonic acid is present in the state, in which the aralkyl phosphonic acid is adsorbed or bonded primarily to the surfaces of the phosphor particles via the polar group described above. The aralkyl phosphonic acids have the advantages over organic acid compounds, such as carboxylic acids and sulfonic acids, in that the adsorption force with respect to the metal surfaces is strong, and the aralkyl phosphonic acids are not apt to undergo desorption from the metal surfaces. Therefore, it is presumed that the aralkyl phosphonic acid is firmly adsorbed to the surfaces of the phosphor particles contained in the phosphor layer of the third radiation image storage panel in accordance with the present invention, and the surfaces of the phosphor particles are brought into the state such that the surfaces are covered with the aromatic rings, and the like. As a result, the affinity of the phosphor particles with respect to the resin constituents will presumably be enhanced, and the stability of the dispersed state of the phosphor particles will presumably be enhanced.
Also, it is considered that, by virtue of the effects of the aralkyl phosphonic acid described above, the phosphor particles and the binder have a strong mutual action. Therefore, it is considered that, after the phosphor layer has been formed, the binder does not undergo desorption from the surfaces of the phosphor particles. As a result, durability of the radiation image storage panel against movement operations is capable of enhanced markedly. Furthermore, since the aralkyl phosphonic acid employed in the third radiation image storage panel in accordance with the present invention has a lower susceptibility to water than other organic acid compounds, such as sulfonic acid, a radiation image storage panel having a high water resistance and good durability is capable of being obtained.
In the third radiation image storage panel in accordance with the present invention, ordinarily, the content of the aralkyl phosphonic acid in the phosphor layer should preferably fall within the range of 0.001 to 5 parts by weight per 100 parts by weight of the phosphor particles. Such that a better dispersed state of the phosphor particles may be obtained and the glossiness of the phosphor layer surface may be enhanced, the content of the aralkyl phosphonic acid in the phosphor layer should more preferably fall within the range of 0.005 to 2 parts by weight per 100 parts by weight of the phosphor particles. Also, such that the light emission characteristics may be enhanced markedly, the content of the aralkyl phosphonic acid in the phosphor layer should most preferably fall within the range of 0.01 to 1 part by weight per 100 parts by weight of the phosphor particles. If the content of the aralkyl phosphonic acid in the phosphor layer is lower than 0.001 part by weight per 100 parts by weight of the phosphor particles, it will often become impossible to obtain the effects of the aralkyl phosphonic acid. Even if the content of the aralkyl phosphonic acid in the phosphor layer is higher than 5 parts by weight per 100 parts by weight of the phosphor particles, further enhancement of the dispersed state of the phosphor particles cannot be expected.
By way of example, the first, second, and third radiation image storage panels in accordance with the present invention may be produced with the process described below. The process for producing the radiation image storage panels containing the stimulable phosphor will be described hereinbelow. In cases where the first, second, and third radiation image storage panels in accordance with the present invention are utilized as the intensifying screens for use in the radiography, the intensifying screens may be produced in a similar manner.
The first, second, and third radiation image storage panels in accordance with the present invention may be produced with a process comprising the steps of:
i) subjecting the phosphor particles, the organic phosphorus compound or the aralkyl phosphonic acid, the binder, and the like, to dispersing processing in order to prepare a coating composition,
ii) performing application of the coating composition in order to form a phosphor sheet,
iii) locating the phosphor sheet, which has been formed with the dispersing processing and the application of the coating composition, on a substrate, and
iv) compressing the phosphor sheet, which has been located on the substrate, at a temperature equal to at least a softening temperature or a melting temperature of the binder, the phosphor sheet and the substrate being thereby adhered to each other.
Alternatively, the first, second, and third radiation image storage panels in accordance with the present invention may be produced with a process comprising the steps of:
i) subjecting the phosphor particles to surface treatment with the organic phosphorus compound or the aralkyl phosphonic acid,
ii) subjecting the surface-treated phosphor particles and the binder to dispersing processing in order to prepare a coating composition,
iii) performing application of the coating composition in order to form a phosphor sheet,
iv) locating the phosphor sheet, which has been formed with the dispersing processing and the application of the coating composition, on a substrate, and
v) compressing the phosphor sheet, which has been located on the substrate, at a temperature equal to at least a softening temperature or a melting temperature of the binder, the phosphor sheet and the substrate being thereby adhered to each other.
In an example of the technique for enhancing the dispersibility of the phosphor particles by the addition of the organic phosphorus compound or the aralkyl phosphonic acid, the organic phosphorus compound or the aralkyl phosphonic acid may be dissolved or dispersed in an organic solvent having a low boiling temperature, and the phosphor particles may be introduced into and mixed with the resulting solution or the resulting dispersion. Thereafter, the organic solvent may be removed, and the surface-treated phosphor particles may thereby be prepared. Also, the phosphor layer may be formed by use of the phosphor particles, and the radiation image storage panel may be produced by utilizing the phosphor layer. In a different example of the technique for enhancing the dispersibility of the phosphor particles by the addition of the organic phosphorus compound or the aralkyl phosphonic acid, the organic phosphorus compound or the aralkyl phosphonic acid may be introduced into a coating composition for the formation of the phosphor layer, preferably in the state in which the organic phosphorus compound or the aralkyl phosphonic acid has been dissolved or dispersed in a portion of a solvent utilized for the preparation of the coating composition for the formation of the phosphor layer, and the resulting mixture may be subjected to kneading and dispersing processing.
As the binder utilized in the first, second, and third radiation image storage panels in accordance with the present invention, a thermoplastic resin, which has elasticity at normal temperatures and which exhibits fluidity when being heated, should preferably be employed. Examples of the thermoplastic resins include a polyurethane, a polystyrene type of elastomer, a polyolefin type of copolymer, a polyester, a polyamide, a polybutadiene, an ethylene-vinyl acetate copolymer, a vinyl chloride type of copolymer, natural rubber, fluorine rubber, a polyisoprene, a chlorinated polyethylene, a butadiene type of copolymer, and silicone rubber.
Of the above-enumerated thermoplastic resins, elastomers are more preferable. As the elastomers, ordinarily, elastomers having a softening temperature or a melting temperature falling within the range of 30xc2x0 C. to 300xc2x0 C. are employed. Elastomers having a softening temperature or a melting temperature falling within the range of 30xc2x0 C. to 150xc2x0 C. are particularly preferable.
In particular, polyurethane resins are preferable. Polyurethane resins having functional groups, such as a sulfonic acid group, a carboxylic acid group, and a phosphoric acid group are more preferable.
The binder described above, the phosphor, and the solvent are mixed together sufficiently. In this manner, the coating composition, in which the phosphor has been dispersed uniformly in a binder solution, is prepared.
Examples of the solvents include lower alcohols, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, and n-butyl alcohol; chlorine atom-containing hydrocarbons, such as methylene chloride and ethylene chloride; ketones, such as acetone, methyl ethyl ketone, and methyl isopropyl ketone; esters of lower fatty acids with lower alcohols, such as methyl acetate, ethyl acetate, and butyl acetate; ethers, such as dioxane, ethylene glycol monoethyl ether, and ethylene glycol monomethyl ether; and mixtures of two or more of the above-enumerated solvents.
The mixing ratio of the binder to the phosphor in the coating composition varies for different characteristics desired for the radiation image storage panel, different kinds of the phosphors, and the like. Ordinarily, the mixing ratio of the binder to the phosphor is selected from the range between 1:1 and 1:100 (weight ratio). The mixing ratio of the binder to the phosphor should preferably be selected from the range between 1:8 and 1:40 (weight ratio).
The coating composition may also contain various additives, such as plasticizers for enhancing the binding force between the binder and the phosphor in the phosphor layer after being formed. Examples of the plasticizers utilized for such purposes include phosphoric acid esters, such as triphenyl phosphate, tricresyl phosphate, and diphenyl phosphate; phthalic acid esters, such as diethyl phthalate and dimethoxyethyl phthalate; glycolic acid esters, such as ethylphthalylethyl glycolate and butylphthalylbutyl glycolate; and polyesters of a polyethylene glycol with an aliphatic dibasic acid, such as a polyester of triethylene glycol with adipic acid, and a polyester of diethylene glycol with succinic acid.
The coating composition, which contains the phosphor and the binder and has been prepared in the manner described above, is uniformly applied onto a surface of a temporary substrate for the formation of a sheet. In this manner, a coating film of the coating composition is formed. The operation for applying the coating composition onto the temporary substrate may be performed by utilizing ordinary coating means, such as an extrusion coater, a slide coater, a doctor blade, a roll coater, or a knife coater.
The temporary substrate may be constituted of a material selected from a glass plate, a metal plate, various kinds of materials, which are employed as substrates for intensifying paper (or an intensifying screen) in the conventional radiography, and materials known as substrates for radiation image storage panels. Examples of the materials for the temporary substrates include films of resins, such as cellulose triacetate, a polyethylene terephthalate, a polyamide, a polyimide, and a polycarbonate; metal sheets, such as an aluminum alloy foil and a stainless steel foil; paper, such as ordinary paper, baryta paper, resin-coated paper, pigment paper containing a pigment, such as titanium dioxide, and paper sized with a polyvinyl alcohol, or the like; and plates or sheets of ceramic materials, such as alumina, zirconia, magnesia, and titania.
The coating composition for the formation of the phosphor layer is applied onto the temporary substrate and dried. The dried coating film is separated from the temporary substrate, and the phosphor sheet acting as the phosphor layer of the radiation image storage panel is thereby formed. Therefore, before the coating composition for the formation of the phosphor layer is applied onto the temporary substrate, a releasing agent is applied onto the surface of the temporary substrate, such that the phosphor sheet having been formed on the temporary substrate can easily be separated from the temporary substrate.
Thereafter, besides the phosphor sheet having been formed in the manner described above, a substrate of the radiation image storage panel is prepared. The substrate may be constituted of a material selected from the same materials as those for the temporary substrate, which is utilized for the formation of the phosphor sheet.
In conventional radiation image storage panels, such that the binding strength between the substrate and the phosphor layer may be enhanced, or such that the sensitivity of the radiation image storage panel may be enhanced or an image having good image quality (with respect to sharpness and graininess) may be obtained with the radiation image storage panel, a high-molecular weight substance, such as a polyester copolymer or an acrylic resin copolymer, is applied onto the surface of the substrate, on which surface the phosphor layer is to be overlaid, in order to form an adhesive properties imparting layer, or a light reflecting layer constituted of a light reflecting substance, such as titanium dioxide, a light absorbing layer constituted of a light absorbing substance, such as carbon black, or the like, is formed on the surface of the substrate, on which surface the phosphor layer is to be overlaid. In the radiation image storage panels in accordance with the present invention, various such layers may be formed on the substrate. The layer constitution may be selected arbitrarily in accordance with the characteristics which the radiation image storage panels should have, and the like.
Also, as described in Japanese Unexamined Patent Publication No. 59(1984)-200200, such that an image having a high sharpness may be obtained, fine concavities and convexities may be formed on the surface of the substrate, on which surface the phosphor layer is to be overlaid. (In cases where the adhesive properties imparting layer, the light reflecting layer, the light absorbing layer, or the like, is formed on the surface of the substrate, on which surface the phosphor layer is to be overlaid, fine concavities and convexities may be formed on the surface of the layer formed on the substrate.)
The phosphor sheet, which has been obtained with the dispersing processing and the application of the coating composition, is located on the substrate. The phosphor sheet, which has been located on the substrate, is then compressed at a temperature equal to at least a softening temperature or a melting temperature of the binder. In this manner, the phosphor sheet and the substrate are adhered to each other.
Examples of compression apparatuses, which may be utilized for the compression processing in the processes for producing the radiation image storage panels in accordance with the present invention, include known compression apparatuses, such as a calender roll apparatus and a hot press. With the calender roll apparatus, the compression processing is performed by locating the phosphor sheet, which has been obtained with the dispersing processing and the application of the coating composition, on the substrate, and the phosphor sheet having been located on the substrate is passed at a predetermined speed between rollers, which has been heated to a temperature equal to at least the softening temperature or the melting temperature of the binder. However, the compression apparatuses, which may be utilized for the compression processing in the processes for producing the radiation image storage panels in accordance with the present invention, are not limited to the compression apparatuses described above and may be selected from various other compression apparatuses, with which the sheet of the type described above can be heated and compressed. The pressure under which the compression processing is performed should preferably be at least 5 MPa.
In ordinary radiation image storage panels, a transparent protective film for physically and chemically protecting the phosphor layer is overlaid upon the surface of the phosphor layer, which surface is opposite to the surface in contact with the substrate. The radiation image storage panels in accordance with the present invention should preferably be also provided with the transparent protective film.
The transparent protective film may be constituted of a fluorine resin copolymer; a cellulose derivative, such as cellulose acetate or nitro cellulose; or a transparent synthetic resin, such as a polymethyl methacrylate, a polyvinyl butyral, a polyvinyl formal, a polycarbonate, a polyvinyl acetate, or a vinyl chloride-vinyl acetate copolymer. The transparent protective film may be formed by applying a solution, which contains one of the above-enumerated materials in an appropriate solvent, onto the surface of the phosphor layer. Alternatively, the transparent protective film may be formed by preparing a protective film forming sheet and adhering the protective film forming sheet to the surface of the phosphor layer by use of an appropriate adhesive agent. The protective film forming sheet may be constituted of a plastic sheet formed from a polyethylene terephthalate, a polyethylene naphthalate, a polyethylene, a polypropylene, a polyvinylidene chloride, or a polyamide; or a transparent glass plate. Ordinarily, the thickness of the protective film should preferably fall within the range of approximately 0.1 xcexcm to approximately 20 xcexcm.
Further, such that the sharpness of the obtained image may be enhanced, a colored layer, which absorbs the stimulating rays and does not absorb the light emitted by the stimulable phosphor, may be added at least to one of the layers described above. (The colored layer is described in, for example, Japanese Patent Publication No. 59(1984)-23400.)
The present invention will further be illustrated by the following non-limitative examples.